linux-kernel - Re: [PATCH v6 5/6] mm: vmscan: Avoid split during shrink_folio

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Message-ID: <47eccc1b-71a2-43f3-8d44-7d26c6ddd4f0@arm.com>
Date: Mon, 8 Apr 2024 10:31:58 +0100
From: Ryan Roberts <ryan.roberts@....com>
To: David Hildenbrand <david@...hat.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Huang Ying <ying.huang@...el.com>,
 Gao Xiang <xiang@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
 Yang Shi <shy828301@...il.com>, Michal Hocko <mhocko@...e.com>,
 Kefeng Wang <wangkefeng.wang@...wei.com>, Barry Song <21cnbao@...il.com>,
 Chris Li <chrisl@...nel.org>, Lance Yang <ioworker0@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Barry Song <v-songbaohua@...o.com>
Subject: Re: [PATCH v6 5/6] mm: vmscan: Avoid split during shrink_folio_list()

On 05/04/2024 11:42, David Hildenbrand wrote:
> On 03.04.24 13:40, Ryan Roberts wrote:
>> Now that swap supports storing all mTHP sizes, avoid splitting large
>> folios before swap-out. This benefits performance of the swap-out path
>> by eliding split_folio_to_list(), which is expensive, and also sets us
>> up for swapping in large folios in a future series.
>>
>> If the folio is partially mapped, we continue to split it since we want
>> to avoid the extra IO overhead and storage of writing out pages
>> uneccessarily.
>>
>> THP_SWPOUT and THP_SWPOUT_FALLBACK counters should continue to count
>> events only for PMD-mappable folios to avoid user confusion. THP_SWPOUT
>> already has the appropriate guard. Add a guard for THP_SWPOUT_FALLBACK.
>> It may be appropriate to add per-size counters in future.
>>
>> Reviewed-by: David Hildenbrand <david@...hat.com>
>> Reviewed-by: Barry Song <v-songbaohua@...o.com>
>> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
>> ---
>>   mm/vmscan.c | 17 +++++++++++------
>>   1 file changed, 11 insertions(+), 6 deletions(-)
>>
>> diff --git a/mm/vmscan.c b/mm/vmscan.c
>> index 00adaf1cb2c3..ffc4553c8615 100644
>> --- a/mm/vmscan.c
>> +++ b/mm/vmscan.c
>> @@ -1223,11 +1223,12 @@ static unsigned int shrink_folio_list(struct list_head
>> *folio_list,
>>                       if (!can_split_folio(folio, NULL))
>>                           goto activate_locked;
>>                       /*
>> -                     * Split folios without a PMD map right
>> -                     * away. Chances are some or all of the
>> -                     * tail pages can be freed without IO.
>> +                     * Split partially mapped folios right
>> +                     * away. We can free the unmapped pages
>> +                     * without IO.
>>                        */
>> -                    if (!folio_entire_mapcount(folio) &&
>> +                    if (data_race(!list_empty(
>> +                        &folio->_deferred_list)) &&
> 
> Please violate the 80 chars rule where reasonable.
> 
> "
> Statements longer than 80 columns should be broken into sensible chunks, unless
> exceeding 80 columns significantly increases readability and does not hide
> information.
> "
> 
> 
> Likely some of that code here should be refactored into readable sub-functions.
> 
> codingstyle also mentions
> 
> "The answer to that is that if you need more than 3 levels of indentation,
> you're screwed anyway, and should fix your program."
> 

Boo! :)

Yeah, no problem, I'll sort this.